X-ray transmissive optical mirror apparatus

ABSTRACT

A mirror is reflective to light and transmissive to x-rays. The mirror has a continuous mirror surface and an x-ray aperture within a body portion of the mirror proximate the continuous mirror surface that is transmissive to x-rays.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/101,790filed on Apr. 8, 2005, which is a continuation-in-part of applicationSer. No. 11/077,524 filed on Mar. 9, 2005, entitled “Apparatus, System,and Method for High Flux, Compact Compton X-ray Source,” which claimsthe benefit of the following provisional applications: application Ser.No. 60/560,848 filed on Apr. 9, 2004, application Ser. No. 60/560,864,filed on Apr. 9, 2004; application Ser. No. 60/561,014, filed on Apr. 9,2004; application Ser. No. 60/560,845, filed on Apr. 9, 2004; andapplication Ser. No. 60/560,849, filed on Apr. 9, 2004, the contents ofeach of which are hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was supported in part by a grant from the NationalInstitutes of General Medical Sciences, National Institutes of Health,Department of Health and Human Services, grant number 4 R44 GM066511-02.The U.S. Government may have rights in this invention.

FIELD OF THE INVENTION

The present invention is generally directed towards optical mirrors foruse in x-ray systems, such as mirrors used in Compton backscatteringx-ray systems.

BACKGROUND OF THE INVENTION

Synchrotron x-ray radiation sources are of interest for many differentfields of science and technology. A synchrotron x-ray radiation sourcehas a wavelength that is tunable. Intense x-ray beams with wavelengthsmatched to the atomic scale have opened new windows to the physical andbiological world. Powerful techniques such as x-ray diffraction andscattering are further enhanced by the tunability of synchrotronradiation that can exploit the subtleties of x-ray spectroscopy.

High flux synchrotrons are typically implemented as centralizedfacilities that use large magnetic rings to store high-energy electronbeams. As an illustrative example, a conventional third generationsynchrotron may have a diameter of over 100 meters and utilize a 2-7 GeVbeam, which combined with insertion devices such as undulator magnetsgenerate 1 Angstrom wavelength x-ray radiation.

The large physical size, high cost, and complexity of conventionalsynchrotrons have limited their applications. For example, in manyuniversities, hospitals, and research centers there are limitations onfloor space, cost, power, and radiation levels that make a conventionalsynchrotron impractical as a local source of x-ray radiation. As aresult, there are many medical and industrial applications that havebeen developed using synchrotron radiation that are not widely usedbecause of the unavailability of a practical local source of synchrotronradiation having the necessary x-ray intensity and spectral properties.

Research in compact synchrotron x-ray sources has led to several designproposals for local x-ray sources that use the effect of Comptonscattering. Compton scattering is a phenomenon of elastic scattering ofphotons and electrons. Since both the total energy and the momentum areconserved during the process, scattered photons with much higher energy(light with much shorter wavelength) can be obtained in this way.

One example of a Compton x-ray source is that described in U.S. Pat. No.6,035,015, “Compton backscattered collimated x-ray source” by Ruth, etal., the contents of which are hereby incorporated by reference. FIG. 1shows the system disclosed in U.S. Pat. No. 6,035,015. The x-ray sourceincludes a compact electron storage ring 10 into which an electronbunch, injected by an electron injector 11, is introduced by a septum orkicker 13. The compact storage ring 10 includes c-shaped metal tubes 12,15 facing each other to form gaps 14, 16. An essentially periodicsequence of identical FODO cells 18 surround the tubes 12, 15. As iswell known, a FODO cell comprises a focusing quadrupole 21, followed bya dipole 22, followed by a defocusing quadrupole 23, then followed byanother dipole 24. The magnets can be either permanent magnets (verycompact, but fixed magnetic field) or electromagnetic in nature (fieldstrength varies with external current). The FODO cells keep the electronbunch focused and bend the path so that the bunch travels around thecompact storage ring and repetitively travels across the gap 16. As anelectron bunch circulates in the ring and travels across a gap 16, ittravels through an interaction region 26 where it interacts with aphoton or laser pulse which travels along path 27 to generate x-rays 28by Compton backscattering. The metal tubes may be evacuated or placed ina vacuum chamber.

In the prior art Compton x-ray source of U.S. Pat. No. 6,035,015 apulsed laser 36 is injected into a Fabry-Perot optical resonator 32. Theresonator may comprise highly reflecting mirrors 33 and 34 spaced toyield a resonator period with a pulsed laser 36 injecting photon pulsesinto the resonator. At steady state, the power level of the accumulatedlaser or photon pulse in the resonator can be maintained because anyinternal loss is compensated by the sequence of synchronized input laserpulses from laser 36. The laser pulse repetition rate is chosen to matchthe time it takes for the electron beam to circulate once around thering and the time for the photon pulse to make one round trip in theoptical resonator. The electron bunch and laser or photon pulses aresynchronized so that the light beam pulses repeatedly collide with theelectron beam at the interaction region 26.

Special bending and focusing magnets 41, 42, and 43, 44, are provided tosteer the electron bunch for interaction with the photon pulse, and totransversely focus the electron beam inside the vacuum chamber in orderto overlap the electron bunch with the focused waist of the laser beampulse. The optical resonator is slightly tilted in order not to blockthe x-rays 28 in the forward direction, FIG. 3. The FODO cells 18 andthe focusing and bending magnets 41, 42 and 43, 44 are slotted to permitbending and passage of the laser pulses and x-ray beam into and out ofthe interaction region 26. The electron beam energy and circulationfrequency is maintained by a radio frequency (RF) accelerating cavity 46as in a normal storage ring. In addition, the RF field serves as afocusing force in the longitudinal direction to confine the electronbeam with a bunch length comparable to the laser pulse length.

In the prior art Compton x-ray source of U.S. Pat. No. 6,035,015 theelectron energy is comparatively low, e.g., 8 MeV compared with 3 GeVelectron energies in conventional large scale synchrotrons. In a storagering with moderate energy, it is well-known that the Coulomb repulsionbetween the electrons constantly pushes the electrons apart in alldegrees of freedom and also gives rise to the so-called intra-beamscattering effect in which electrons scatter off of each other. In priorart Compton x-ray sources the laser-electron interaction is used to cooland stabilize the electrons against intra-beam scattering. By insertinga tightly focused laser-electron interaction region 26 in the storagering, each time the electrons lose energy to the scattered photons andare subsequently re-accelerated in the RF cavity they move closer inphase space (the space that includes information on both the positionand the momentum of the electrons), i.e., the electron beam becomes“cooler” since the random thermal motion of the electrons within thebeam is less. This laser cooling is more pronounced when the laser pulseinside the optical resonator is made more intense, and is used tocounterbalance the natural quantum excitation and the strong intra-beamscattering effect when an intense electron beam is stored. Therefore,the electron beam can be stabilized by the repetitive laser-electroninteractions, and the resulting x-ray flux is significantly enhanced.

Conventional Compton x-ray sources have several drawbacks that haveheretofore made them impractical in many applications. In particular,prior art compact Compton x-ray sources, such as that described in U.S.Pat. No. 6,035,015, are not sufficiently bright x-ray sources fornarrowband applications, such as protein crystallography or phasecontrast imaging. Narrowband applications (also known as “monochromatic”applications), are applications or techniques that commonly use amonochromater to filter or select a narrow band of x-ray energies froman incident x-ray beam. As an illustrative example, monochromatorstypically select less than 0.1% of the relative energy bandwidth. As aresult, narrowband applications not only benefit from a source with ahigh total x-ray source but an x-ray flux that is comparatively brightwithin a narrow bandwidth.

The x-ray beam of prior art Compton x-ray sources, such as thatdescribed in U.S. Pat. No. 6,035,015, has a lower brightness thandesired due in part to the large energy spread caused by theelectron-laser interaction. The brightness is also less than desiredbecause the optical power level that can be coupled into and stored inthe Fabry-Perot cavity between mirrors 33 and 34 for use in Comptonbackscattering is less than desired, due to a number of limitations onthe control, stability, and losses in different elements of the opticssystem. Additionally, U.S. Pat. No. 6,035,015 has the optical mirror 34offset from the path of the x-rays to reduce x-ray absorption, resultingin the optical beam being a few degrees off from a true 180 degreebackscattering geometry, which significantly reduces Comptonbackscattering efficiency.

Therefore, what is desired is a compact Compton x-ray source withincreased brightness and efficiency that is suitable for narrowbandsynchrotron radiation applications.

SUMMARY OF THE INVENTION

A mirror is reflective to light and transmissive to x-rays. The mirrorhas a mirror diameter and an associated mirror thickness. The mirrorincludes a first surface polished to form a continuous mirror surfacefor reflecting light. An x-ray aperture within a body portion of themirror underneath the continuous mirror surface comprises a region ofthe body portion transmissive to x-rays within a selected energy range.

One embodiment of an apparatus for reflecting light and transmittingx-rays, comprises: an optical substrate having a first diameter and anassociated minimum thickness, the optical substrate including a firstsurface polished to form a continuous mirror surface and the minimumthickness selected such that the optical substrate is at least partiallytransmissive to x-rays; an optically reflective coating deposited on themirror surface, the optically reflective coating being transmissive tox-rays; at least one mechanical support substrate providing mechanicalsupport to the optical substrate; an x-ray aperture within the at leastone mechanical support substrate including a region of the at least onemechanical support substrate of reduced thickness that is transmissiveto x-rays having a second beam diameter less than the first diameter.

Another embodiment of an apparatus for reflecting light and transmittingx-rays in a Compton backscattering x-ray system comprises: a mirrorhaving a mirror diameter and an associated mirror thickness, the mirrorincluding a first surface polished to form a continuous mirror surfacefor reflecting light in an optical resonator of the Comptonbackscattering x-ray system; an optically reflective multi-layerdielectric stack coating disposed on the mirror surface that istransmissive to x-rays; and an x-ray aperture within a body portion ofthe mirror having reduced x-ray attenuation such that the mirror is atleast partially transmissive to x-rays generated by the Comptonbackscattering x-ray system.

BRIEF DESCRIPTION OF THE FIGURES

The invention is more fully appreciated in connection with the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a block diagram of a prior art Compton x-ray source;

FIG. 2 is a block diagram of a Compton x-ray source in accordance withone embodiment of the present invention;

FIG. 3 illustrates the electron-photon interaction at the interactionpoint;

FIG. 4A illustrates a portion of an ejection/injection cycle in which anew electron bunch is injected;

FIG. 4B illustrates a portion of an ejection/injection cycle in whichthe new electron bunch and the old electron bunch are being deflected bythe kicker;

FIG. 4C illustrates a portion of an ejection/injection cycle in whichthe old electron bunch has been dumped and the new injection bunch hasbeen directed into an orbit in the electron storage ring to replace theold electron bunch;

FIG. 5A illustrates the location of the electron bunch and the photonpulse at a first time;

FIG. 5B illustrates the location of the electron bunch and the photonpulse at a second time;

FIG. 5C illustrates the location of the electron bunch and the photonpulse at a third time;

FIG. 6 is a block diagram illustrating functional equivalent elements ofa mirror;

FIG. 7 is a side profile of a mirror reflective to light andtransmissive to x-rays in accordance with one embodiment of the presentinvention;

FIG. 8 is a top view of a mirror reflective to light and transmissive tox-rays in accordance with one embodiment of the present invention;

FIG. 9 is a cross-sectional view of the mirror of FIG. 8 along line A-A;and

FIG. 10 is an exploded cross-sectional view illustrating a portion ofthe fabrication process of the mirror of FIG. 8.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates a compact x-ray synchrotron source 200 havingfeatures facilitating the generation of nearly monochromatic,collimated, tunable, high-flux beam of x-rays suitable for many x-rayapplications in accordance with one embodiment of the present invention.

X-ray synchrotron source 200 includes an injector 205 portion of thesystem having elements 211, 212, 213, and 214 for generating andinjecting an electron bunch into an electron storage ring 220 via septum225. An electron bunch is a group of electrons that, for example, has aspatial density envelope, energy and momentum distribution, and temporalpulse length. An electron source 210 produces an electron bunch using anRF gun 212 by striking a photocathode material, such as copper ormagnesium, with a pulsed laser 211. A short linear acceleratorsection(s) 213 accelerates the electron beam to the full energy desiredin the ring to obtain a desired x-ray energy for Compton scattering atapproximately 180 degrees with photons having a selected wavelength. Forexample, the electron energy necessary for 1 Å radiation, using a 1 μmwavelength laser pulse, is 25 MeV. A transport line 214 consisting offocusing and steering magnets prepares the electron bunch for injectioninto the electron storage ring 220.

An injected electron bunch is steered using a septum magnet 225 near theclosed orbit trajectory of the electron storage ring 220. The bunchtrajectory is then aligned to the proper storage ring orbit by a fastdeflector magnet (kicker) 226. In one embodiment kicker 226 uses acompact set of distributed deflector magnets to reduce the peak power,and hence complexity, of the drive electronics.

An ejector 290 portion of the system includes kicker 226 and shieldedbeam dump 227. Kicker 226 ejects an existing, circulating pulse tolocally shielded beam dump 227. The locally shielded electron dump 227provides a controlled means to minimize unwanted radiation emission. Inone embodiment locally shielded electron dump 227 includes a collimatoror aperture at the entrance of the dump to intercept an ejected electronbunch. Each dump (ejection) of an electron bunch generates radiationthat, if unshielded, would raise background radiation levels in theregion around x-ray synchrotron source 200. Radiation shielding (notshown) associated with locally shielded electron dump 227 is sized andlocated to reduce background radiation levels. As described below inmore detail, septum 225, kicker 226, and shielded beam dump 227 arepreferably designed to permit a mode of operation in which the electronbunch is regularly (e.g., periodically) refreshed (e.g., simultaneousinjection of a new electron bunch and ejection of an old electron bunch)in order to reduce the time-averaged energy and momentum spread of theelectron bunch compared with steady-state operation. Localized radiationshielding of shielded beam dump 227 is thus desirable to facilitateregular refreshment of the electron bunch while maintaining acceptableaverage background radiation levels.

The electron storage ring 220 includes focusing and steering elementsfor guiding and maintaining an electron bunch in a stable, closed orbit.A magnet lattice composed of quadrupole focusing magnets 228 and dipolebending magnets 229 contain and steer the beam in a closed-loop orbit.The dipole magnets, together with intervening quadrupole magnet(s) 230may form achromatic optical systems to facilitate injection and beamoptics matching.

The beam is kept tightly bunched by an RF cavity 231. On one side of thering is a straight section in which the electron beam is transverselyfocused to a small spot by a set of quadrupole magnets 232. This smallspot is called the interaction point (IP) 233 (also sometimes known asthe “interaction region”) and is coincident with the path of a focusedlaser pulse created by optics system 250. The beam dynamics of theelectron bunches are preferably optimized to achieve a stable orbit withacceptable degradation. Beam position monitors may be included tomonitor the beam trajectory of electron bunches along the ring.

Optics system 250 generates photons that collide with electrons in theinteraction point 233. In one embodiment, photons collide with electronsin a 180 degree backscattering geometry. A pulsed laser source 234drives an optical resonator 245 formed by the optical cavity associatedwith mirrors 239, 240, 241(a), and 241 b. The optical resonator 245 hasan optical path between mirrors 242 and 240 that is coaxial with aportion of the storage ring 220 through the interaction point 233. Thepulsed laser source 234 includes elements needed to condition the laserfor coupling power efficiently to optical resonator 245. A feedbacksystem may consist of a local controller 235, such as a high bandwidthpiezo-electric mirror assembly, to track the laser frequency to that ofthe optical resonator 245. Such systems may use an optical modulator 236to implement a frequency discrimination technique that produces anappropriate error signal from a reflected signal 242.

An input mirror 239 is made purposely transmissive to enable opticalpower to enter the cavity from the drive laser 234. The inputtransmission value should be ideally matched to the total internalcavity losses in order to efficiently couple power (impedance matched).The cavity mirrors are curved with values appropriate to produce a smallspot of similar size to the focused electron beam at the IP 233.

The optical resonator 245 may be a conventional cavity, such as aFabry-Perot cavity. However, in one embodiment the optical resonator 245is a ring cavity. Additional optical elements (not shown) may be used toadjust the position of one or more of the mirrors 242, 240, 241 a, and241 b.

In one embodiment mirrors 239, 240, 241 a, 241 b are configured to forma ‘bowtie’ ring cavity as shown. The bowtie configuration is a travelingwave ring, which has increased stability over a standing waveFabry-Perot optical cavity. Note that in a bowtie configuration thatthere are two (phase/time shifted) pulses that are simultaneouslycirculated in resonator 245. If a bowtie configuration is used, x-raysystem 200 is preferably designed to synchronize the collision of eachof the two circulating phase/time shifted photon pulses with theelectron bunch.

An optical resonator 245 including a bowtie ring cavity affords severaladvantages. The geometry of a bowtie cavity permits a long separation,L, between mirrors 239, 240 within a reasonable total area. This is due,in part, to the fact that the bowtie configuration permits the width, W,between mirrors 239 and 241 a to be less than the length, L, separatingmirrors 239 and 240, particularly if the angular deflection provided byeach mirror is selected to be comparatively small. A long separation, L,facilitates shaping photon pulses to have a narrow waist near IP 233that increases at mirrors 242 and 240. The long separation, L,facilitates selecting the optical elements to form a narrow opticalwaist at interaction point 233 where the opposing electron beam islikewise transversely focused. Additionally, because the two mirrors239, 240 on each side of the waist can be separated by a large distance,L, in a bowtie configuration, the bowtie configuration permits asubstantial increase in the beam size on these mirrors that in turnlowers the optical power per unit area or risk of damage. That is, along distance between mirrors 239, 240 permits a narrow optical waist atinteraction point 233 and a comparatively wide optical profile atmirrors 239, 240. In one embodiment, the ratio of transverse beam sizebetween the mirrors and the waist is approximately 100:1 which leads toa reduction in power density at mirrors 239, 240 on the order of 10000:1for an axially symmetric TEM00 mode. Since one limiting factor on thecirculating power in optical resonator 245 is mirror damage, thereduction in power levels at the mirror facilitates storing a highoptical power level within optical resonator 245.

A large distance between mirrors 239, 240 is also advantageous to steeran electron beam in to and out of the IP using dipole magnets 229 suchthat the collision may occur at an optimal 180 degree backscatteredgeometry. A very small or zero crossing angle between electron andphoton pulses maintains a high interaction efficiency yet allows thepulses to remain relatively long for improved stability. A naturalconsequence of this 180 degree backscattering geometry is that generatedx-rays will follow a path coincident with the electron path trajectoryat the IP 233 and consequently be directed toward and through oneoptical mirror 240.

Using high-quality spherical mirrors in such geometries may require alocal controller (not shown) to adjust astigmatism on a subset ofmirrors to guarantee small interaction spots in both transversedimensions at the IP 233. Another local control on the overall cavitylength, for instance with a set of piezo-electric elements on one mirror(not shown), is required to set the cavity round-trip time (or pathlength) to be harmonically related to the round-trip time (path length)of the electrons in the ring in order to properly synchronizecollisions.

A polarization controller 237 may be included to rapidly change theoptical polarization properties of the optical pulse, which in turnaffects the polarization of generated x-rays. Such a controller may beimplemented using an electrical Pockels cell or mechanically controlledwaveplates.

Matching optics 238 manipulate the transverse mode profile to match theeigenmode of the cavity. Flat turn mirrors may also be included to alignthe incident beam to the optical cavity axis. The matching optics areillustrated as transmissive lenses, although for high powerapplications, using all reflective optics may be preferred to preserveoptical mode quality.

In one embodiment optical resonator 245 is a high-finesse cavity andpulsed laser source 234 is a frequency stabilized mode-locked laser thatresonantly drives the high-finesse cavity of optical resonator 245. Inone embodiment the pulsed laser source 234 is a mode locked laser thatis actively stabilized to efficiently drive a high-finesse externalresonator 245. The mode-locked laser system resonantly drives theenhancement cavity to build-up a high power laser pulse. The lasertracks the resonance of the optical cavity through active feedback, suchas deriving an error signal by measuring the phase of the promptreflection to that of some small leaked cavity power 242. A photodiodeor other high bandwidth detector (not shown) may be used on thereflected signal 242 from the cavity for feedback or wavefrontdiagnostics.

An optical resonator 245 formed from a high finesse cavity is capable ofstoring large photon densities. The optical cavity gain and powerstorage performance has an intrinsic limit given by the total losses ofthe cavity and mirror power handling. Advances in optical coatings havepushed mirror scatter and absorption losses down to part per millionlevels. These low losses together with higher surface damage thresholdsare compatible with an average circulating power on the order of amegawatt or more using commercially available solid-state laser power.However, a high finesse cavity has a narrow bandwidth, which means thathigh optical power levels within a high finesse cavity can only beefficiently achieved if pulsed laser source 234 is a frequencystabilized laser source, such as a mode-locked laser, with a narrow,controlled bandwidth that is matched to the high finesse cavity. As oneexample, the combination of a frequency stabilized mode locked laser anda high finesse cavity permits a pulsed gain enhancement in excess of10,000, which reduces the laser source complexity and cost needed for aCompton scattering x-ray source due to the increase in optical powerlevel.

In the backscattering geometry illustrated in FIG. 2 an electron bunch270 and a photon pulse 280 interact at interaction point 233 andgenerate x-rays 243 that pass through mirror 240. The efficiency of theCompton backscattering process is improved for a true 180 degreebackscattering process compared with the off-axis geometry illustratedin prior art FIG. 1. However, a conventional laser mirror 240 adds asubstantial x-ray attenuation. A 180 degree backscattering geometry thusprovides a net increase in x-ray brightness only if the x-rayattenuation of mirror 240 is sufficiently low. Optical mirror 240 isthus preferably low loss for photon pulses and also transmissive forx-rays. For example, mirror 240 may be a high quality optical mirrorformed from a multi-layer dielectric reflective stack deposited on asubstrate. The dielectric layers and the substrate are preferablyselected to transmit, with little attenuation, hard x-rays with energies≧5 keV. For example, optical mirrors made from Tantalum (Ta₂O₅/SiO₂) orZirconium (ZrO₂/SiO₂) dielectric layers contain absorption energies inthe hard x-ray spectrum although the overall attenuation due to thecoatings should preferably not exceed ˜10% for multi-layer stackmirrors. A preferred combination of dielectric coating materials for adielectric stack is alternating layers of Titania/Silica (TiO₂/SiO₂).The K-edge is at 5 keV, which makes the x-ray transmission efficient for˜7 keV and higher x-ray energies. Another possible combination ofdielectric materials for >1 μm wave-length is Silicon/Silica (Si/SiO₂).Exemplary substrate materials include Beryllium, Silicon, and Sapphire.Another possible substrate is to coat a super-polishable (˜1 Å rms)substrate, preferably Silicon, with CVD diamond to improve mechanicalrigidity and thermal conductivity.

A timing system 260 synchronizes the timing of different components.Ring timing 262 generates signals for controlling the ring orbitfrequency for electron bunches. Injection and ejection timing 264generates signals for controlling the injection of new electron bunchesand the simultaneous ejection of old electron bunches. The refreshmentof the electron bunches is preferably performed periodically accordingto a selected period (e.g., at a rate of 10 to 200 Hz). Periodicrefreshment has the benefit over other scheduling protocols that itfacilitates synchronization. However, it will be understood thatinjection and ejection timing 264 may define a refreshment scheduleother than that of a fixed period. Laser and resonator timingsynchronization 266 generates timing signals for synchronizing opticalsystem 240 to the electron bunches.

FIG. 3 illustrates the backscatter geometry near the interaction point233. The electron bunch 270 and the laser (photon) pulse 280 have avertical height (i.e., radial profile) across the length of an electronbunch 270. Focusing elements within storage ring 220 focus the electronbunch 270 whereas optical elements focus the laser pulse 280. At theinteraction point 233 the electron bunch and the laser pulse preferablya similar radial waist radius, σ, although to generate particular x-raybeam qualities the beam waists may be selected to be purposelydifferent. The electron bunch 270 and the laser pulse 280 travel inopposite directions towards the interaction point 233 and produce aburst of x-rays 243 as their envelopes pass through each other.Fundamental principles of electron focusing and optical focusing may beused to calculate the three-dimensional profile of the electron bunch270 and the laser pulse 280. The electron bunch and the laser pulse arepreferably formed and focused to have similar profiles and temporallengths selected to optimize Compton backscattering. Note that theradial diameter of an electron bunch or a photon pulse increases alongits length due to fundamental principles of electron and opticalfocusing. Generally, increasing the temporal length of an electron bunchor a photon pulse increases the radial spread. A substantial increase inradial diameter can reduce the average brightness of the x-ray source.As described below in more detail, it is desirable to select thetemporal length of the electron bunch and the laser pulse such that eachpulse length results in expansion that is less than the Rayleigh range,or depth of focus.

The repetition rate for collisions is determined by the ring geometry.For example, a 4 meter circumference would give an interaction rate of75 MHz corresponding to the inverse of the time it takes for an electronbunch to make one revolution around the ring. The x-rays are directed ina narrow cone in the direction of the electron beam. They can be focusedusing conventional x-ray optics down to the source image size, forinstance ˜60 μm diameter. Gross x-ray energy can be tuned by adjustingthe electron beam energy, and fine-tuning can be accomplished by amonochromator crystal or filter adjustment.

The general configuration as described above operates with a similarphoton flux up to x-ray energies of many tens of keV, and can be scaledto gamma ray energies as well.

As previously described, in one embodiment the injector 205 and ejector290 regularly refreshes the electron bunch in the storage ring toimprove the beam quality. For example, in a periodic refreshmentprocess, after an electron bunch has completed a pre-selected number oftrips around the ring (e.g., one million turns) the “old” electron bunchis ejected and a “new” electron bunch is injected to take its place. Inthe example of a system having an interaction rate of 75 MHz, refreshingat a rate of 60 Hz would permit each electron bunch to make 1.25 millionturns before being ejected. Note that this mode of operation is incontrast to conventional synchrotron operation in which the synchrotronis traditionally operated in a steady-state mode as long as practicalfor beam stability and other considerations.

FIGS. 4A-4C illustrate aspects of a refreshment process in which an oldelectron bunch is ejected while a new electron bunch is simultaneouslyinjected. Referring to FIG. 4A, an electron bunch 400 that has completeda pre-selected number of turns enters septum 225. A new electron bunch410 is synchronized to enter kicker 226 at a time selected such that thenew electron bunch 410 will replace the old electron bunch 400.Referring to FIG. 4B, kicker 226 is turned on during the refresh processto apply a magnetic field that shifts the path of an electron bunchwithin kicker 226. Note that the two electron bunches 400 and 410 haveslightly different initial trajectories. Both the old electron bunch 400and the new electron bunch 410 are spatially shifted by the magneticfield applied by kicker 226. Referring to FIG. 4C, kicker 226 applies asufficient magnetic field to shift new electron bunch 410 into a propertrajectory to enter a stable orbit in electron storage ring 220 as thenew stored electron bunch. Conversely, old electron bunch 400 has itstrajectory shifted to exit the electron storage ring 220 in dump 227.Kicker 226 is then turned off until the next refresh cycle.

When a new electron bunch 410 is first injected into the electronstorage ring 220, it has a comparatively narrow energy and momentumspread and can be tightly focused. However, with each subsequentrevolution around the ring attributes of the electron bunch affectingx-ray emission degrade according to a time response. In particular, theenergy and momentum of the electron bunch may increase to equilibriumlevels for times greater than a degradation time (e.g., a degradationtime may correspond to an exponential time constant to approach e⁻¹ of afinal saturation value). For example, the time response for intra-beamscattering (with radiation damping) is a response in which theintra-beam scattering begins at some initial rate of increase with therate of increase exponentially decaying over time until the intra-beamscattering reaches a saturation level for time periods sufficientlylong, e.g., for times greater than some saturation time constant.Additionally, any other degradation mode that may exist in the systemwould be expected to have its own associated time response anddegradation time constant. However, in many applications, intra-beamscattering is expected to be the dominant degradation mechanism.Intra-beam electron beam scattering increases with each revolution andeventually reaches a saturation level through photon cooling (e.g.,radiation damping) over a time scale that is commonly on the order of afraction of a second, as described in more detail in R. J. Loewen. ACompact Light Source: Design and Technical Feasibility Study of aLaser-Electron Storage Ring X-Ray Source. PhD thesis, StanfordUniversity, Stanford, Calif., June 2003, the entire contents of whichare hereby incorporated by reference.

Intra-beam scattering increases the transverse emittance (the product ofthe spot size and the angular divergence) and also the longitudinalemittance (the product of the bunch length and energy spread). Anincrease in transverse emittance is undesirable because an increasedtransverse emittance reduces the x-ray brightness. This is because thetransverse emittance is inversely proportional to the output x-rayquality or brightness, where the brightness can be expressedmathematically in terms of photons, area, solid angle (solidangle), andbandwidth as brightness=photons/(area*solidangle*bandwvidth). Anincrease in longitudinal emittance (energy spread) increases thebandwidth of the x-rays, which is undesirable for narrowbandapplications because it means that the usable flux within a desired“monochromatic” bandwidth is decreased. As an illustrative example, inthe prior art system of FIG. 1, the equilibrium value of intra-beamscattering and quantum excitation lead to an order of magnitude increasein energy spread, and commensurate reduction in brightness, from theinitial injected electron beam parameters.

Selecting the refresh rate to increase x-ray brightness involves atradeoff between several different considerations. The upper limit onrefresh rate is limited by system considerations such as electron bunchinjection power, design complexity, synchronization issues, andshielding concerns (since each beam dump will generate undesirableradiation). The lower limit on refresh rate is bounded by the desire tooperate in a transient mode in which time-averaged x-ray brightness(within a selected bandwidth) is improved compared with steady stateoperation. In particular, the minimum refresh rate may be selected suchthat the period of the rate is less than the time for the electron bunchto degrade to a steady-state saturation level. For example, the minimumrefresh rate may be selected to be less than a degradation time constantfor degrading to the steady state. As a result, the time-averagedelectron bunch attributes improve, resulting in a correspondingimprovement in the time-averaged x-ray attributes.

As an illustrative example, for a particular application the minimumrefresh rate may be selected to achieve a desired increase in the levelof x-ray brightness within a desired bandwidth compared with operatingin steady-state. As one example, a simulation by one of the inventors ofthe present application indicates that the energy and momentum of anelectron bunch may approximately double over a time scale of 0.01seconds. Over a longer period of time (e.g., a significant fraction of asecond), steady state is reached with a decrease in brightness of aboutan order of magnitude. Thus, in one embodiment the refresh rate isselected to maintain the degradation below a pre-selected level, such asa factor of two or three increase in degradation. Consequently,periodically refreshing the electron bunch at a fraction of the dominanttime constant, such as at rates greater than about 10 Hz (e.g., 10 to200 Hz) is sufficient to operate in a transient (non-steady state) modethat improves x-ray brightness and reduces the bandwidth of the x-rays.Thus, in this illustrative example it can be understood that dramaticincreases in brightness can be achieved by periodically refreshing theelectron bunch. Although the improvement in brightness is useful for avariety of applications, the corresponding reduction in bandwidth ofemitted x-rays also makes periodic refreshment of particular interestfor narrowband synchrotron x-ray application where the x-ray source musthave a high flux within a comparatively narrow bandwidth.

While the electron bunch may be refreshed at a fixed rate, moregenerally the electron bunch may be refreshed according to a schedule.For example, the schedule may include a fixed rate of refreshment, avariable rate of refreshment, an adjustable rate of refreshment, orother scheduling schemes. However, in many applications a fixed refreshrate is the simplest schedule to implement using commercially availableelectronics.

Regularly refreshing the electron bunch also provides other benefits tosystem design of a compact Compton x-ray source. In particular, thenumber of components is reduced compared with a steady-state design. Forexample, a steady-state design conventionally includes components tostabilize the beam over long time periods (e.g., hours or days), whichincreases the number of components that are required. For example, aconventional compact Compton x-ray source designed to operate in steadystate for extended time periods must include components to stabilize thelarge equilibrium energy spread of the beam, such as throughchromaticity correction or a low momentum compaction lattice.

FIGS. 5A-5C are illustrations at three different times illustrating howthe electron bunch and optical pulse are timed to meet at the IP 233. Asshown in FIG. 5A, at some initial time, an electron bunch 500 traversesthe kicker 226. In the optical cavity, a first optical pulse 510 isdirected towards mirror 240. A second optical pulse 520 is directedtowards mirror 241 b. As shown in FIG. 5B, at a second time, theelectron bunch 500 has moved partially towards the IP while the firstoptical pulse 510 strikes mirror 240 and the second optical pulse 520strikes mirror 241 b. As illustrated in FIG. 5C, at a third time, theelectron bunch 500 and optical pulse 510 are directed in oppositedirections and interact at the IP 233.

Referring again to FIG. 2, in one embodiment timing system 260 adjustsring timing 262, periodic injection and ejection timing 264, and laserand resonator timing and synchronization 266. A preferred embodimentemploys a set of master frequency sources that are phase-locked to highprecision and distributed to the device subsystems, thus circumventingcomplicated or costly timing feedback systems. In one embodiment, anintegrated master frequency generator internally phase locks to each ofthe required frequencies using phase locked coaxial resonatoroscillators. In one embodiment a base frequency F1 (˜100 MHz) representsthe electron ring circulation time and interaction rate. An RF frequencyat some multiple of that rate F2=n*F1 (e.g. n=16) is fed to one or moreRF cavities in the electron storage ring 220 to stabilize longitudinaldynamics. Furthermore, another harmonic multiple of the ring frequencyF3=m*F1 (e.g. m=30) may be used to power the RF gun 212 and acceleratingstructures. The particular choice of frequencies depends on source costsand desired beam parameters. In one embodiment a high-power S-band (˜3GHz) pulsed microwave source is used to power the injector system, andthen derive suitable ring circulation frequencies from sub-harmonics.

Timing system 260 coordinates the electron bunch injection and ejectioncycles to maintain electron bunches in the storage ring withinacceptable design parameters for absolute timing, energy spread andbunch length. The injection/ejection cycle is preferably performed atsome integer multiple of the AC line frequency in order to reducepulse-to-pulse timing (phase) jitter generated from high power RFsystems. These timing pulses are generated, for example, from a fast 60Hz zero-crossing detector logically ‘AND’ ed with the desired ringcirculation frequency F1. The pulsed injector system may, for example,operate in the 10 to 200 Hz frequency range (“re-injection” rate).

The photocathode of RF gun 212 requires phase stability between the RFpowering the device (F3) and the timing or phase of the incident laser212 used to create the electron bunch (F1). Laser pulses derived from amode-locked laser 234 are stabilized at F1 using a phase-locked loop.One of these pulses is selected to seed an amplifier (not shown) thatthen delivers a frequency-converted (UV) laser pulse to the photocathodeat the proper RF phase (F3). A phase shifter in an RF line tunes therelative phase offset.

The timing system 260 may also generate all sample clocks required byreal-time servo controls in the optical resonator 245, including opticalcavity length and alignment control systems for proper phasing orsynchronization between the electron and photon pulses to assure optimalcollision efficiency at the interaction point (IP). The highest samplingrate for feedback loops is determined by the ring frequency (˜100 MHz).However, by individually stabilizing both the electron ring circulationand the optical cavity length from one master frequency source, F1, onlyphase drifts need to be adjusted once the proper relative phase is set.

Selection of the pulse length of the electron bunches and photon pulsesinvolves several tradeoffs. Selecting extremely short pulses (e.g., lessthan one picosecond pulses) facilitates obtaining tight focus ofelectron bunches and photon pulses and results in the highest peak x-rayoutput. However, the use of extremely short pulses has the drawback thattiming synchronization and control is difficult to achieve usingcommercially available electronics. Pulse lengths less than about onepicosecond require specialized, ultrafast electronics to achievesub-picosecond synchronization and phasing. Additionally, jitter is aproblem when the pulse lengths are less than about one picosecond. Inparticular, it is difficult to maintain a jitter budget of less thanabout one picosecond for many commercially available electronic andoptical components.

In one embodiment, the electron bunches and photon pulses have pulselengths from about one picosecond to several tens-of-picoseconds. Usingtens-of-picosecond-long electron bunches and photon pulses relaxes therequired timing synchronization to similar levels. The absolute timingor phase correction, arising from thermal or mechanical drifts, requiresonly a slow feedback loop and may be optimized by monitoring x-ray flux.More importantly, longer electron pulses reduce the peak current of thebeam which helps reduce the onset of instabilities or other beamdynamics that degrade electron beam quality. The use of longer opticalpulses also optimizes storing high power in the high-gain optical cavityby reducing the effect of dispersion (carrier envelope offset) betweenthe mode-locked laser and cavity modes.

The upper limit on the pulse lengths is limited by depth of focusissues, which can reduce the average x-ray flux if the pulse lengths areselected to be greater than several tens of picoseconds. The x-ray fluxfor a 180 degree backscatter geometry is only weakly sensitive to thelength of the electron bunches as long as they are some fraction of thedepth of focus, or Rayleigh range. The reduction in intensity is due tothe “hourglass effect” which begins to dramatically reduce effectiveluminosity at bunch lengths beyond the depth of focus. A similarhourglass effect occurs for the photon pulses. The hourglass effect isillustrated in FIG. 3. There are practical limits to the achievableminimum waist sizes for both the electron beam and optical beam. For theelectron beam, smaller than 30 micron waists require stronger and morecomplicated magnets (and more room to put them) that also lead tostronger chromatic effects that affect beam stability. Achieving under20 micron waists is extremely difficult. For the optical beam, a smallwaist in a resonant cavity implies working close to an instabilitypoint. For practical and achievable waists, the corresponding depth offocus is typically many millimeters long. For a 1 micron laser and afocus of 30 microns (rms), the Rayleigh range is typically about 10 mm,which corresponds to approximately 30 picosecond long laser pulses andelectron bunches.

In one embodiment, jitter timing budgets are also selected to facilitaterecovering optimum electron bunch and photon pulse timing after eachrefreshment of the electron bunch. Returning again to FIGS. 4A-4C, inthe ideal case the new electron bunch 410 would exactly replace the oldelectron bunch 400 with no change in timing that would reduce theCompton backscattering efficiency at IP 233. However, in a practicalsystem the new electron bunch 410 replaces the old electron bunch 400nearly synchronously, i.e., within a jitter timing budget that ispreferably small enough that jitter in the timing of the electron bunchcreated by the refreshment process will not deleteriously disrupt theoperation of the timing system 260. In particular, it is desirable thatthe jitter timing budget is small enough that timing system 260 does nothave to perform a radical resynchronization of the photon pulse timingafter each refreshment of the electron bunch. As an illustrativeexample, the jitter timing budget may be selected to be less than thetemporal length of an electron bunch to reduce the complexity and costof designing a control system to rapidly recover optimum electron bunchand photon pulse timing after a refreshment of the electron bunch.

FIG. 6 illustrates some of the design issues associated with designing amirror 240 having low x-ray transmission losses and optical losses inthe part-per-million (ppm) range. Mirror 240 can be modeled as havingthe equivalent functional elements of an optical reflector 605 coupledto an x-ray attenuator 610. Input (incident) x-rays enter and exit x-rayattenuator 610. Incident (input) light strikes optical reflector 605 andreflected light is returned. Increases in the reflectivity of opticalreflector 605 increases the optical power stored in optical resonator245, resulting in an increase in x-ray flux generated by Comptonbackscattering. However the x-ray transmission losses of mirror 240 mustalso be taken into account because increases in the x-ray transmissionlosses of equivalent x-ray attenuator 610 decreases the net x-ray outputflux.

The design of mirror 240 thus involves two considerations, namelyoptimizing optical reflectivity while also minimizing x-ray transmissionlosses to an acceptable level. For example, one prior art mirror designis to place a hole extending completely through the entire mirror. Thehole allows x-rays to pass through the mirror. However, light can alsoexit through the hole. The hole thus results in the equivalent reflector605 having a reduced reflectivity (since some of the light also escapesthrough the hole). To mitigate losses, one may use a higher orderoptical transverse mode (that has a central null coincident with thehole) in the optical resonator 245. Notwithstanding the more complicatedoptical matching of the laser to resonator (compared with using thefundamental TEM00 mode), higher order modes have significant losses forpractical sized holes for x-ray passage. These additional losses arebeyond the ppm levels required for high gain cavities. As such, the useof these high order modes are suitable for only low finesse cavities.Thus, prior art mirror designs are generally unable to simultaneouslyachieve both a high optical reflectivity and a low x-ray transmissionloss.

The x-ray transmission losses of equivalent x-ray attenuator 610 aredetermined by the x-ray transmission loss though the portion of mirror240 that the x-rays pass through. Generally speaking, each portion ofthe mirror of a given length, L, and x-ray attenuation coefficient, γ,will cause an exponential decrease in x-ray flux of exp(−γL). The totalx-ray transmission loss through the mirror may be calculated byintegrating the losses through each portion of the mirror that the x-raybeam passes through.

The reflectivity of equivalent optical reflector 605 is determinedlargely by the polishing process and optical coatings used to fabricatemirror 240. However, conventional approaches to optimizing mirrorreflectivity result in high x-ray losses for a variety of reasons.First, an extremely high reflectivity mirror should be opticallycontinuous over a diameter sufficient that it intercepts substantiallyall of the optical power of an incoming optical beam (e.g., to achieve areflectivity of 99.99% requires that the mirror intercept slightly morethan 99.99% of the optical power of the incoming beam). This means thatin a 180 degree backscattering geometry that to achieve highreflectivity the x-ray beam must pass through the mirror with, in anexact 180 degree geometry, the x-ray beam being coaxial with the centerof the optical beam such that the x-ray beam passes through the centerof mirror 240, resulting in high x-ray losses.

Additionally, extremely low-loss mirrors (with ppm losses) requireextremely low scattering losses. As a result, extremely low loss mirrorstypically have the mirror surface polished to a surface roughness ofabout 1 Angstrom root mean square (rms), what is commonly known in theart as “super-polishing.” Super-polishing is typically required toachieve scattering losses on the order of 1 ppm. Super-polished mirrorsurfaces may be fabricated by commercial vendors, such as ResearchElectro-Optics of Boulder, Colo. USA. However, a high qualitysuper-polished mirror fabrication process requires a minimum ratio, M,of the diameter, D, relative to an optical substrate thickness, T, ofD/T<M to provide mechanical support and stability during polishing. Asecondary consideration is that D/T must be selected to facilitatemechanical stability of mirror 240 after polishing. If the D/T ratio istoo large, mirror 240 may warp after polishing due to residual stress,strain, or heating. Note that even comparatively small deformations(e.g., a fraction of an optical wavelength) may degrade thecharacteristics of mirror 240. As a result of these polishing andmechanical considerations a conventional super-polished mirror typicallyrequires the mirror surface to be formed on a thick substrate having acorresponding high x-ray transmission loss.

As an illustrative example, a D/T ratio commonly used in the mirrorpolishing art for high quality polishing of spherical mirrors withsub-Angstrom surface roughness is 5:1, i.e., the substrate thicknessshould be at least about 20% of the diameter of the substrate tofacilitate high quality polishing. Thus, in an implementation in which Dis comparatively large the thickness, T, must also increase, whichcorrespondingly increases x-ray losses. As an illustrative example, if Dis 50 millimeters, T would conventionally have to be at least 10millimeters to be compliant with 5:1 polishing rule for high qualitypolishing. However, a substrate thickness of several millimeters resultsin high x-ray losses for conventional substrate materials capable ofbeing super-polished, such as a silicon substrate.

FIG. 7 is a side view of a mirror 740 for reflecting light andtransmitting x-rays with reduced x-ray transmission losses according toone embodiment of the present invention. Mirror 740 may be used in placeof mirror 240 to improve x-ray flux output of a 180 degree Comptonbackscattering system. Mirror 740 has a mirror surface 705 formed in asupporting substrate 710. Mirror surface 705 has an optically reflectivecoating 715 formed on it with an average thickness, t. Opticallyreflective coating 715 has a high reflectivity at a selected opticalwavelength range and a low optical loss. One criterion for low mirrorloss is that the mirror surface 705 must be continuous and extend oversubstantially all of the optical profile at the mirror plane, i.e., itcannot have any holes and it must extend to a diameter, D, sufficient tointercept substantially all of the power of the optical beam strikingmirror 740.

A low optical loss mirror 740 also requires polishing the mirror towithin tight tolerances. The surface figure, which is the deviation ofthe mirror surface 705 from an ideal contour, is preferably less than atenth of a wavelength. Additionally, residual defects must also becomparatively low. For example, the surface quality is at least 10-5,and preferably 5-2, in a conventional scratch-dig test. A dig is adefect on a polished optical surface that is nearly equal in terms ofits length and width. A scratch is a defect on a polished opticalsurface whose length is many times its width. The nomenclature 10-5indicates the average diameter of the digs to be 0.05 mm and the averagelength of a scratch is 0.10 mm.

Substrate 710 has an associated x-ray loss attenuation coefficient perunit distance. As is well known, materials with a high atomic-Z numbertypically have high x-ray attenuation coefficients. X-ray attenuation isthus reduced by selecting comparatively low-Z materials. Consequentlysubstrate 710 is preferably formed from one or more low-Z materials. Onepossibility is low-Z metals, such as aluminum, titanium, or beryllium.However, some low-Z materials, such as Beryllium, are difficult topolish to sub-Angstrom rms surface roughness. Beryllium, for example, isboth mechanically too soft and too toxic to be a desirable substratematerial. Silicon, diamond, and sapphire are stiff low-Z substratematerials with comparatively low x-ray attenuation coefficients.Additionally, techniques to super polish sapphire and silicon arewell-developed in the polishing art. Hybrid low-Z multi-layersubstrates, such as sapphire-on-silicon and diamond-on-silicon, are alsopossible substrate choices to achieve a structure with a surface layercapable of being super-polished (e.g., silicon) on top of a base layerproviding desirable mechanical and thermal properties (e.g., diamond).

Additionally, optical coating 715 has an associated average x-rayattenuation coefficient per unit distance. In one embodiment, opticalcoating 715 is a multi-layer dielectric stack coating, such as a stackof alternating quarter-wavelength thick high and low index materials. Ina quarter-wavelength stack each layer has a physical thickness of λ0/4n,where λ0 is the free-space wavelength and n is the refractive index.

The dielectric materials of optical coating 715 are preferably selectedto reduce x-ray losses. In particular there is a correlation between theatomic Z value and the index of refraction such that higher indexdielectric materials tend to have higher x-ray attenuation. Low indexmaterials, such as silica (SiO₂) or Alumina (Al₂O₃) tend to be low-Zmaterials with low x-ray attenuation. However, high index materialsformed from high-Z metallic oxides, such as Tantalum (Ta₂O₅) or Hafnium(HaO₂) tend to have K-edge x-ray absorption edges within common desiredx-ray energies of 6 to 18 keV. As is well known in the art of optics,the reflectivity of a multi-layer dielectric stack of quarter-wavelengthlayers increases with the number of layers in the stack. Thus, toachieve an extremely high reflectivity, such as a reflectivity of0.9999, would conventionally require a substantial number of layers. Ifone or both of the layers in the dielectric stack are formed from high-Zmaterials the total x-ray attenuation can be substantial. Consequently,in one embodiment, a dielectric stack is formed from a quarterwavelength stack of two materials that are each selected to have acomparatively low x-ray attenuation over a broad energy range, such as adielectric stack of TiO₂/SiO₂ which is acceptable for 6 keV and higherx-rays, or over a selected range of x-ray energies, such as Nb₂O₅/SiO₂which is acceptable for x-ray energies of 6 to 18 keV. In one embodimentthe dielectric layer stack is chosen to have a total x-ray loss nogreater than 10%. As an illustrative example, a stack of thirty-twoTiO₂/SiO₂ layers, each a quarter wavelength thick for a free-spaceoptical wavelength of 1.06 microns, has a reflectivity of at least0.9999 and a total x-ray transmission loss at 12 keV of only 4.03%.Thus, with such a dielectric stack the light impinging on the mirror isreflected whereas the dielectric stack is highly transmissive to x-rays.

The total (net) x-ray flux from a 180 degree Compton backscatteringsystem increases with increasing reflectivity of mirror 740 (due to thecapability to develop a higher optical power level at the interactionpoint 233) but is also reduced by x-ray transmission losses in mirror240. As previously described, optical coating 715 is transmissive tox-rays with the appropriate selection of materials and total thickness(e.g., x-ray transmission losses less than 10%). However, x-raytransmission losses in substrate 710 can still be substantial. In a 180degree Compton backscattering geometry the x-ray flux passes throughoptical coating 715 and then into and through substrate 710. To a firstorder approximation, the total average distance that the x-ray passesthrough substrate 710 is the substrate thickness, T. A more accurateapproximation, however, is to average the x-ray flux over substrate 710to take into account small deviations in thickness caused by thecurvature of mirror surface 705. For example, at the center of mirrorsurface 705 the mirror curvature results in a small decrease inthickness, 6.

As previously described, in one embodiment the optical resonator isdesigned to increase the optical mode waist at the mirrors. While thex-rays generated by Compton backscattering will increase in diameterwith distance from IP 233, typically the diameter of the x-ray cone atan exit mirror will be only a small fraction of the diameter of the exitmirror. In one embodiment an x-ray aperture region 718 having low x-raytransmission loss is formed within a body portion 775 of mirror 740. Thex-ray aperture region 718 is transmissive to x-rays with a maximumpre-selected x-ray transmission loss. For example, in one embodiment thetotal x-ray transmission losses of x-rays passing through the opticalcoating 715 and the x-ray aperture region 718 is selected to be nogreater than 50%.

As illustrated by phantom lines, in one embodiment x-ray aperture region718 is a region in which the mirror has a reduced thickness. Forexample, in one embodiment a cavity within substrate 710 having adiameter, d, is formed that is larger than an x-ray beam diameter at anexit mirror 740 and with a depth T0 relative to back surface 780selected so that the residual (average) effective thickness that thex-ray beam passes through mirror 740 is reduced in x-ray aperture region718 to a thickness T1 (offset by any mirror curvature) selected toachieve a desired maximum x-ray transmission loss. As a result, thex-ray transmission losses are reduced compared with a substrate ofuniform thickness T0. X-ray aperture region 718 may be formed using avariety of fabrication processes, such as etching a cavity prior tomirror polishing or etching a cavity after mirror polishing.

Mirror 740 preferably has the values of D and d selected to optimizeoptical reflectivity and x-ray transmissivity for a particular opticalmode waist size at the mirror and a particular x-ray beam diameter. Notethat the thicker portions of body portion 775 provide mechanical supportfor the comparatively thin mirror section in x-ray aperture region 718.The mechanical support provided by the thicker portions of body portion775 permits the thickness of x-ray aperture region 718 in a diameter, d,to be thinner than that that could be achieved if body portion 775 had auniform thickness across the entire diameter D. For a given substratematerial, conventional modeling techniques may be used to analyzeparticular combinations of D, d, T, T0, and T1 for a particular set ofoptical and x-ray beam parameters. For example, mechanical and thermalmodeling may be performed to determine ranges with acceptable mechanicalcharacteristics of the mirror surface proximate x-ray aperture region718. This may include, for example, analyzing mechanical stability,stress, strain, deformation, and thermal effects for particularselections of D, d, T, T0, and T1. The effect of optical heating ofmirror 740 from optical absorption may also be modeled. The modeling mayinclude analyzing different substrate materials. The effect of cavitydiameter, d, relative to mirror diameter D, may also be modeled todetermine an optimum ratio that provides desired mechanical properties.Additionally, the modeling may include analyzing substrates having auniform composition or substrates formed from different layers.Empirical studies may be used to further select an optimum mirror designfor a particular application that is compatible with a high qualitymirror fabrication process and that has a combination of a low totalx-ray transmission loss and acceptable mirror characteristics. Whilex-ray aperture region 718 may be created by forming an empty cavity inbody portion 775, more generally it will be understood that a cavity mayalso be partially filled if desired to increase mechanical stability.For example, if x-ray aperture 718 has a comparatively large diameter itmay be desirable to provide additional support within an associatedcavity, such as support struts, a thin cap region, a low x-rayabsorption material, or other features to provide additional mechanicalstability in regions of body 775 having a reduced thickness.

FIG. 8 is a top view of a mirror 840 having reduced x-ray absorption inaccordance with one embodiment of the present invention. FIG. 9 is across-sectional view along line A-A. A first portion 1 of mirror 840comprises a thick mechanical substrate 810 that includes a cavity 818extending through the entire thickness of thick mechanical substrate 810to form an x-ray aperture. A second portion 2 of optical mirror 840includes an optical substrate 825 having a mirror surface 805 polishedwith a selected radius of curvature. Second portion 2 may also include athin mechanical substrate 830 disposed between thick mechanicalsubstrate 810 and optical substrate 825. A multi-layer stack reflector(not illustrated in FIG. 9) is formed on mirror surface 805. Asillustrated by phantom lines, an optical mode having an optical modewaist strikes mirror surface 805. An x-ray cone passes through cavity818.

FIG. 10 is an exploded view illustrating a fabrication process forforming mirror 840. A first portion 1 comprises thick substrate 810 ofdiameter D and thickness T having a D/T ratio selected to providemechanical support for mirror polishing. For example, the thickness, T,may be several millimeters. A D/T ratio of D/T<5 is desirable forminimizing surface figure errors in a super-polishing process. D isdetermined by the size of the optical beam waist at the mirror surface.As is well known, a lateral optical mode in an optical resonator has abeam waist and a small amount of optical power in a region outside ofthe waist that decrease rapidly with distance. The mirror diameter isthus preferably larger than the optical waist, w, by a margin selectedto achieve a desired optical loss. For example, calculations indicatethat to have a mirror loss of less than 5 ppm that the mirror shouldhave a diameter related to the mode waist, w, of at least 2.5w andpreferably 3w. A clear aperture hole having a diameter d is fabricatedinto substrate 810 to create cavity 818. The ratio of d is preferablymuch less than D. In one embodiment the d:D ratio is 1:5. Thicksubstrate 810 is preferably an optical quality substrate with goodmechanical stiffness and thermal stability, such as a silicon substrate.The thick substrate is preferably optically polished piano-flat on bothof its sides.

Second portion 2 comprises optical substrate 825 having a startingthickness of t0 selected to accommodate material losses duringpolishing. The backside of optical substrate 825 may be coated or bondedto stiffer material for mechanical support, such as a thin mechanicallayer 830 of chemical vapor deposited diamond. For example, in oneembodiment thin mechanical layer 830 is a diamond layer of 100-200microns thickness deposited onto an optical substrate 825 that is asilicon substrate. A thin mechanical layer 830 fabricated from a diamondfilm improves the mechanical strength and thermal heat dissipation ofthe portion of optical substrate 825 that it supports.

The first portion 1 and the second portion 2 are then bonded together.As one example, silicate bonding may be used to bond the first portionto the second portion. Silicate bonding processes utilize silicate basedbonding chemistry to bond two materials together, such as silicon andsilicon dioxide materials. A silicate bonding process provides alow-stress interface that has high strength, low interface bondingstress, and is also compatible with use of the mirror in an ultra-highvacuum system. Silicate bonding processes are typically formed onsurfaces composes of silicon and/or oxides of silicon, such as siliconor silicon dioxide surfaces. In the case that the first portion and thesecond portion are both silicon substrates, the silicate bonding may bedirectly performed on the substrates. However, a thin silicon dioxidelayer, such as a silicon dioxide layer of about one micron in thickness,may be deposited on a mechanical substrate 830 to facilitate silicatebonding. This permits a mechanical substrate 830 comprised of a diamondfilm to be bonded to a thick substrate 810 comprised of silicon.

After substrate bonding the optical substrate 825 is polished to form ahigh quality optical mirror surface having a desired contour (e.g., aspherical or parabolic mirror surface) and with a pre-selected residualminimum thickness proximate the x-ray aperture. The residual minimumthickness of optical substrate 825 is determined by the trade-off ofmechanical support and x-ray transmissivity. For the case that theoptical substrate is composed of silicon and the mechanical substrate830 is composed of diamond, exemplary design ranges include 10 to 100microns residual minimum thickness of the silicon layer and 50 to 200microns for the diamond layer. As illustrative examples for 12 keVx-rays, the combined transmission losses for a 25 micron silicon layeron top of 200 microns of diamond is 18% while a 10 micron layer ofsilicon on top of 100 microns of diamond is 8%. The layer thicknessesand compositions of the optical substrate layer and the mechanicalsupport substrate may be selected to permit the polishing of a nearlyperfect spherical surface. However, it is also possible to utilize asecond stage of polishing to correct minor distortions in the mirrorabout the x-ray aperture. Small surface figure distortions on the orderof up to a few microns may be partially or completely ameliorated by asecond optical differential polishing technique, such asmagnetorheological finishing.

After polishing of the mirror surface is complete, a dielectricmultilayer mirror coating is deposited on the polished mirror surface.The multilayer mirror coating may be deposited with conventional coatingtechniques, such as ion-beam deposition. As previously described, thecomposition and thickness of the multilayer stack is preferably chosento achieve a high optical reflectivity and a low x-ray loss.

While mirrors 740 and 840 have been described in regards to applicationsin Compton backscattering systems, more generally it will be understoodthat mirrors 740 and 840 may be utilized in other applications as well.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that specificdetails are not required in order to practice the invention. Thus, theforegoing descriptions of specific embodiments of the invention arepresented for purposes of illustration and description. They are notintended to be exhaustive or to limit the invention to the precise formsdisclosed; obviously, many modifications and variations are possible inview of the above teachings. The embodiments were chosen and describedin order to best explain the principles of the invention and itspractical applications, they thereby enable others skilled in the art tobest utilize the invention and various embodiments with variousmodifications as are suited to the particular use contemplated. It isintended that the following claims and their equivalents define thescope of the invention.

1-20. (canceled)
 21. A system to generate x-rays by Comptonbackscattering, comprising: an electron storage ring guiding electronsthrough an interaction point disposed along a portion of said electronstorage ring; an optical system generating photon pulses coupled to saidinteraction point, the optical system including an optical resonatorhaving a mirror reflective to said photon pulses and transmissive tox-rays, the mirror including: a mirror body portion configured toprovide mechanical support to a mirror surface, the mirror surface beingcontinuous throughout an entire region encompassed by a first diameterand the mirror body portion having an interior region of reducedthickness within a second diameter so that the mirror body portion is atleast partially transmissive to x-rays, the second diameter being lessthan the first diameter; an optically reflective coating deposited onsaid mirror surface, said optically reflective coating being at leastpartially transmissive to x-rays; and the mirror being reflective tolight throughout the entire region of the mirror surface encompassed bythe first diameter inclusive of the portion of the mirror surface withinthe second diameter, a portion of the mirror within the second diameterfurther being an x-ray aperture for transmission of x-rays through themirror.
 22. The system of claim 21, wherein said optically reflectivecoating comprises a dielectric stack mirror for reflecting light at apre-selected wavelength, said dielectric stack mirror including asequence of dielectric materials of different refractive indices havinga total x-ray loss below a first pre-selected value and a total opticalreflectivity of at least a second pre-selected value.
 23. The system ofclaim 22, wherein said mirror surface is super-polished with a surfaceroughness of less than one Angstrom rms.
 24. The system of claim 22,wherein said dielectric stack mirror comprises a sequence ofquarter-wave thick layers having a total reflectivity of at least about0.9999.
 25. The system of claim 22, wherein said x-ray aperturecomprises a cavity within a mechanical substrate of said mirror to forma region of reduced thickness in said mirror.
 26. The system of claim21, wherein said first diameter is selected so that the mirrorintercepts at least 99.99% of the optical power of an optical mode ofthe optical resonator.
 27. The system of claim 21, wherein said mirrorsurface is polished directly into said mirror body portion.
 28. Thesystem of claim 21, wherein said mirror surface is formed on an opticalsubstrate portion of the mirror body portion.
 29. A system to generatex-rays by Compton backscattering, comprising: an electron storage ringguiding electrons through an interaction point disposed along a portionof said electron storage ring; an optical system generating photonpulses coupled to said interaction point, the optical system including ahigh finesse optical resonator having a mirror reflective to said photonpulses and transmissive to x-rays, the mirror including: a mirror bodyportion configured to provide mechanical support to a mirror surface,the mirror surface being continuous throughout an entire regionencompassed by a first diameter and the mirror body portion having aninterior region of reduced thickness within a second diameter so thatthe mirror body portion is at least partially transmissive to x-rays,the second diameter being less than the first diameter; an opticallyreflective coating deposited on said mirror surface, said opticallyreflective coating being at least partially transmissive to x-rays; andthe system having a 180 degree backscattering geometry with an outputx-ray beam being coaxial with a center of an optical mode impinging themirror surface, the mirror being reflective to light throughout theentire region of the mirror surface encompassed by the first diameterinclusive of the portion of the mirror surface within the seconddiameter, a portion of the mirror within the second diameter furtherbeing an x-ray aperture for transmission of x-rays through the mirror.30. The system of claim 29, wherein said optically reflective coatingcomprises a dielectric stack mirror for reflecting light at apre-selected wavelength, said dielectric stack mirror including asequence of dielectric materials of different refractive indices havinga total x-ray loss below a first pre-selected value and a total opticalreflectivity of at least a second pre-selected value.
 31. The system ofclaim 30, wherein said mirror surface is super-polished with a surfaceroughness of less than one Angstrom rms.
 32. The system of claim 30,wherein said dielectric stack mirror comprises a sequence ofquarter-wave thick layers having a total reflectivity of at least about0.9999.
 33. The system of claim 29, wherein said first diameter isselected so that the mirror intercepts at least 99.99% of the opticalpower of the optical mode.
 34. The system of claim 29, wherein saidmirror surface is polished directly into said mirror body portion. 35.The system of claim 29, wherein said mirror surface is formed on anoptical substrate portion of the mirror body portion.
 36. A methodgenerating x-rays by Compton backscattering, comprising: storingelectrons in an electron storage ring; storing photons in an opticalresonator coupled to a portion of the electron storage ring to generatean x-ray beam via approximately 180 degree Compton backscattering at aninteraction point; utilizing a mirror of the optical resonator tosimultaneously transmit the x-ray beam and reflect photons back into theoptical resonator, the mirror being reflective to light throughout anentire region of a mirror surface encompassed by a first diameter and abody portion of the mirror having a reduced thickness within a seconddiameter less than the first diameter to form an x-ray aperture for thex-ray beam.
 37. The method of claim 36, further comprising expanding anoptical mode of the resonator along an optical path of the opticalresonator from the interaction point to the mirror surface so that atthe mirror surface an optical mode waist of the optical mode is greaterin diameter than the x-ray beam.